Process for preparing aromatic isothiocyanates from aromatic nitro compounds



United States Patent 3,535,364 PROCESS FOR PREPARING AROMATIC ISO-THIOCYANATES FROM AROMATIC NITRO COMPOUNDS Peter H. Scott, Guilford, andHaywood Hooks, In, West Haven, Conn., assignors to Olin Corporation, acorporation of Virginia No Drawing. Filed Oct. 29, 1968, Ser. No.771,603 Int. Cl. C07c 161/04 US. Cl. 260-454 13 Claims ABSTRACT OF THEDISCLOSURE A process for preparing aromatic isothiocyanates whichcomprises reacting an aromatic nitrogen-containing compound such as anaromatic nitro compound, an aromatic nitroso compound, an aromatic azocompound or an aromatic azoxy compound with carbon disulfide and/orcarbonyl sulfide in the presence of potassium hydrosulfide.

This invention relates to an improved process for the preparation ofaromatic isothiocyanates.

Esters of isothiocyanic acid have been previously prepared. They areuseful agricultural chemicals since they have exhibited valuable utilityas fungicides and herbicides. Many of these esters are usefulnematocides and insecticides particularly as moth-proofing agents.Isothiocyanates have also been extensively employed as intermediates inthe preparation of pesticidal and pharmaceutical compounds. Forinstance, they have been reacted with stoichiometric amounts of chlorineto provide N-aryland N-alkyl-S-chloroisothiocarbamoyl chlorides, forexample, as disclosed in Journal of Organic Chemistry, 31, 838 (1966);and these derivatives are useful as herbicides and nematocides.Isothiocyanates also react with a molar excess of chlorine to providethe corresponding isocyanide dichlorides which are known to be usefulpesticides.

A variety of synthetic methods have been previously utilized to obtainthe aforementioned esters. For example, they may be generally preparedby the reaction of primary amines with thiophosgene, but this is not apractical procedure since thiophosgene is not readily available. Some ofthe isothiocyanates have been prepared by the reaction of isocyanateesters with phosphorus pentasulfide, but this is not a general reactionand cannot be utilized in the preparation of all isothiocyanates. Theesters have also been prepared by an involved synthetic route comprisingreacting primary amines with carbon disulfide in the presence ofselected bases to provide salts of dithiocarbamic acids which can thenbe further reacted to the desired isothiocyanates, but this is acomplicated and costly procedure.

It has now been found that aromatic isothiocyanates are provided in goodyield and high purity by reacting. a carbocyclic aromatic nitrogencompound such as an aromatic nitro compound, an aromatic nitrosocompound, an aromatic azo compound, an aromatic azoxy compound, ormixtures thereof with carbon disulfide and/or carbonyl sulfide in ananhydrous system at an elevated temperature in the presence of potassiumhydrosulfide. The process of this invention is a convenient, directonestep procedure for providing aromatic isothiocyanates from cheap,readily available reactants. This process obviates the requirement ofutilizing the previously dis closed tedious multi-step procedures.Addition of potassium hydrosulfide to the reaction mixture not onlyincreases the yield of aromatic isothiocyanate but also reduces thenumber of byproducts, thereby greatly simplifying the recovery procedureand lowering process costs.

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For convenience, the aromatic nitrogen compound reactant of thisinvention will be defined in terms of a carbocyclic aromatic nitrocompound. Corresponding aromatic nitroso, aromatic azo and aromaticazoxy compounds can also be employed. It is to be understood thatproportions of reactants, catalyst, solvent and the like based on nitrogroups in the aromatic nitro compound also represent the same proportionranges per mole of nitrogen-containing group if the aromatic nitrocompound is replaced with an aromatic nitroso, aromatic azo or aromaticazoxy compound.

More in detail, the aromatic nitro compound reactant may be at least oneof a wide variety of aromatic nitro compounds. As used herein, the terma carbocyclic aromatic nitro compound represents those organic compoundshaving at least one nitro group attached directly to an aromatichydrocarbon nucleus such as a benzene, naphthalene, anthracene,phenanthrene and the like. The aromatic hydrocarbon nucleus may alsocontain other ring substituents in addition to the nitro groups. Thusthe term carbocyclic aromatic nitro compound as used herein alsorepresents aromatic hydrocarbons having alkyl, aryl, aralkyl, alkoxy,aryloxy, alkylrnercapto, arylmercapto, halogen, cyano, isocyanato, orisothiocyanate substituents on the aromatic hydrocarbon moiety inaddition to the one or more nitro groups. In general, these additionalring substituents do not inhibit completely the reaction of carbondisulfide or carbonyl sulfide with the nitro groups under the conditionsof the process disclosed herein. Carbon disulfide or carbonyl sulfidemay also react with some of these additional ring substituentsconcurrently with the reaction of the nitro groups, and some of thesesubstituents may impede or retard the desired reaction of CS or COS withthe nitro groups as for instance by introducing a steric hindrancefactor; but invariably some formation of aromatic isothiocyanate occursby the process albeit at a reduced rate or in lower yield.

Thus among the aromatic nitro compounds which may be used as reactantsin the practice of this invention are the various nitrobenzenes,nitronaphthalenes and nitroanthracenes. Also included as usefulreactants are the various nitrobiphenyls, nitrotoluenes, nitroxylenes,nitromesitylenes, nitrodiphenyl alkanes, alkoxynitrobenzenes,nitrodiphenyl ethers, nitropolyphenyl ethers, alkylmercaptonitrobenzenes, nitrodiphenyl thioethers, nitrobenzonitriles, andaromatic nitrohalocarbons.

Illustrative of specific aromatic nitro compounds useful as reacantsare: nitrobenzene, o-dinitrobenzene, m-dinitrobenzene, p-dinitrobenzene,1,3,5-trinitrobenzene, lnitronaphthalene, 2-nitronaphthalene,o-nitrotoluene, mnitrotoluene, p-nitrotoluene, 2,4-dinitrotoluene,2,6-dinitrotoluene, o-nitro-p-xylene, 2-methyl-l-nitronaphthalene,dinitromesitylene, o-nitrobiphenyl, m-nitrobiphenyl, p-nitrobiphenyl,4,4-dinitrobiphenyl, 2,4-dinitrobiphenyl, bis(p-nitrophenyl)methane,o-nitroanisole, m-nitroanisole, p-nitroanisole, 2,4-dinitroanisole,o-nitrophenetole, pnitrophenetole, and 2,4-dinitrophenetole.

Similarly: o-nitrophenyl phenyl ether, m-nitrophenyl phenyl ether,p-nitrophenyl phenyl ether, bis(2,4-dinitrophenyl)ether, bis(pnitrophenyl)ether, o nitrophenyl phenyl thioether, m-nitrophenyl phenylthioether, p-nitrophenyl phenyl thioether, bis(p nitrophenyl)thioether,o nitrophenyl methyl thioether, bis(p nitrophenoxy) ethane, 1 chloro 2nitrobenzene, 1 bromo 2 nitrobenzene, 1 chloro 3 -nitrobenzene, 1 bromo,3 nitrobenzene, 1 chloro 4 nitrobenzene, 1 bromo-4- nitrobenzene, 1fluoro 4 nitrobenzene, 2 chloro-6- nitrotoluene, 2 bromo 6 nitrotoluene,2 fluoro-6- nitrotoluene, 4 chloro 3 nitrotoluene, 1 chloro 2,4-dinitrobenzene, 1 bromo 2,4 dinitrobenzene, 1 fluoro- 2,4dinitrobenzene, 1,4 dichloro 2 nitrobenzene,

1,4 difiuoro 2 nitrobenzene, 1,3,5 trichloro 2 nitro benzene, 1,3,5tribromo 2 nitrobenzene, 1,2 dichloro- 4 nitrobenzene, 1,2,4 trichloro 5nitrobenzene, onitrophenyl isocyanate, m nitrophenyl isocyanate,pnitrophenyl isocyanate, 1 chloro 2,4 dimethoxy-S- nitrobenzene, 1,4dimethoxy 2 nitrobenzene, o nitrobenzonitrile, m nitrobenzonitrile, pnitrobenzonitrile, 3,3 dimethoxy 4,4 dinitrobiphenyl, and 3,3 dimethyl4,4 dinitrobiphenyl may be employed as starting reactants.

Isomers and mixtures of the aforesaid aromatic nitro compounds andsubstituted aromatic nitro compounds may also be utilized in thepractice of this invention as well as homologues and other relatedcompounds. Generally, the starting nitro compound reactants containbetween 6 and about and preferably below about 14 carbon atoms.Compounds which have both nitro and isothiocyanato substituents may alsobe employed as reactants. When aromatic polynitro compounds are utilizedas reactants in this process, considerable amounts of compounds havingboth nitro and isothiocyanato groups are usually provided. Thus, forinstance, when bis(p-nitrophenyl)sulfide is employed as a reactant, 4nitrophenyl 4' isothiocyanatophenyl sulfide is provided. Since theprocess of this invention is conveniently adaptable to batchwise,semi-continuous, or continuous operations, the nitro-isothiocyanatoderivative may be utilized as a starting reactant in a new batchoperation or may simply be directly converted to the bis-isothiocyanatoderivative by recycling in a continuous practice of this process.

While the process is generally applicable to the conversion of any ofthe aforementioned aromatic nitro compounds to aromatic isothiocyanates,included among the preferred reactants to be utilized in this inventionare the nitrobenzenes, both monoand polynitro, including isomericmixtures thereof; the alkylnitrobenzenes, including the various nitratedtoluenes and the nitrated xylenes; the alkoxynitrobenzenes; the nitratedmono-, diand trichlorobenzenes and toluenes; nitrated biphenyl andnitrated diphenylmethane. Other preferred reactants which can beparticularly mentioned include the nitrodiphenyl ethers, thebis(nitrophenoxy)alkanes, and the bis(nitrophenyl) sulfides.

Aromatic nitroso compounds, aromatic azo compounds, and aromatic azoxycompounds are also converted to aromatic isothiocyanates in accordancewith this invention. As described in the preceding discussion relatingto suitable aromatic nitro compound reactants, the aromatic nitroso,aromatic azo and aromatic azoxy compounds may also contain one or moreother substituents on the aromatic ring in addition to the reactivenitroso, azo or azoxy groups.

In the reaction with aromatic mononitro, mononitroso, monoazo ormonoazoxy reactants, it has been found that preferred practice involvesutilizing at least one mole of carbon disulfide or carbonyl sulfide permole of nitrogenous reactant. When a bifunctional reactant (e.g., adinitro compound) is used, the amount of CS or COS is preferablydoubled. Optimum yields of aromatic isothiocyanates are in fact obtainedwhen excess molar amounts of carbon disulfide or carbonyl sulfide areemployed, that is, more than an equirnolar quantity in reaction with amononitro compound for instance. The use of molar excesses is alsoadvantageous in that the CS and COS function as solvents in the reactionsystem.

The function of potassium hydrosulfide in the reaction is not clearlyunderstood, but it appears to have some catalytic effect on thereaction. For this reason, potassium hydrosulfide is referred tothroughout the description and claims as a catalyst, even though it mayalso be a reactant or other agent during the reaction.

As indicated previously, the number of by-products are substantiallyreduced when the reaction is carried out in the presence of potassiumhydrosulfide. As a result, the residue after isolation of the aromaticisothiocyanate,

generally predominates in the corresponding N,N-disubstituted urea,which is easily isolated by well-known chemical techniques. Since thereaction product predominates in the isothiocyanate and the substitutedurea, a significant savings in equipment and operating costs arerealized by the use of potassium hydrosulfide, because of the relativesimplicity in separating and recovering the reaction products.

In carrying out the process of one embodiment of this invention, thearomatic nitro compound and catalyst are placed in a suitable pressurevessel, such as an autoclave, which is equipped with a sparger forfeeding gas or liquid into the bottom thereof. The pressure vessel isalso optionally provided with agitation means as well as cooling andheating means. Preferably, in the case of carbonyl sulfide, after theslurry or solution of catalyst and aromatic nitro compound is placedinto the pressure vessel, it is sealed, and carbonyl sulfide is pumpedinto the pressure vessel through the sparger until the desired amount isadded. Preferably in the case of carbon disulfide, the desired amount ofcarbon disulfide is added as a liquid to the catalyst and aromatic nitrocompound before the pressure vessel is closed. However, if desired thecarbon disulfide can be added in the same manner as described above forthe carbonyl sulfide, which may be liquid and/or gas when added to thereactor. For convenience, the term sulfur compound of carbon will beused throughout the description and claims to include either carbondisulfide, carbonyl sulfide or mixtures there of in any ratio.

After the desired temperature and pressure conditions are obtained, thesulfur compound of carbon may be fed continuously through the spargerinto the suspension of catalyst and aromatic nitro compound during theentire reaction period while maintaining the pressure at the desiredlevel.

The order of mixing the reactants is not critical and may be variedwithin the limitations of the equipment employed. In one embodiment, thearomatic nitro compounds, catalyst, sulfur compound of carbon in liquidform and, if desired, solvent, are charged to a suitable pressure vesselsuch as an autoclave which was previously purged with nitrogen, andwhich is preferably provided with agitation means such as a stirrer oran external rocking mechanism. The operating pressure can be attained byheating and/or by feeding under appropriate temperature conditions thesulfur compound of carbon into the autoclave. The operating pressureafter heating or after feeding the sulfur compound of carbon into theclosed autoclave is in the range between about 30 and about 10,000p.s.i.g., and preferably between about and about 2000 p.s.i.g., butgreater or lesser pressures may be employed if desired.

Generally the quantity of the sulfur compound of carbon in the freespace of the reactor is maintained at a level sufficient to maintain thedesired pressure as Well as to provide reactant for the process, as thereaction progresses. If desired, additional sulfur compound of carboncan be fed to the reactor either intermittently or continuously as thereaction progresses to maintain the pressure within the above range. Thetotal amount of the sulfur compound of carbon added is generally betweenabout 1 and about 50, and preferably between about 2 and about 15 molesof the sulfur compound of carbon per nitro group in the aromatic nitrocompound. Greater or lesser amounts may be employed if desired. Thehighest requirements of the sulfur compound of carbon are generallyutilized in a process in which the gas is added continuously, butsuitable recycle of the gas stream greatly reduces the overallconsumption of the sulfur compound of carbon.

The molar proportion of catalyst to each nitro group in the aromaticnitro compound in the reaction is generally equivalent to between about121000 and about 1:1, and preferably between about 1:100 and about 1:2.However, greater or lesser proportions may be employed if desired.

The reaction between the sulfur compound of carbon with the aromaticnitro compound may be effected in the absence of a solvent, but can alsobe performed in a solvent which is chemically inert to the components ofthe reaction system. Suitable solvents include aliphatic,cycloaliphatic, aromatic solvents such as n-heptane, cyclohexane,benzene, toluene, and xylene, and halogenated aliphatic and aromatichydrocarbons such as dichloromethane, trichloroethylene,perchloroethylene, tetrachloroethane, monochlorobenzene,dichlorobenzene, and chloronaphthalene, as well as tetramethyl urea,dioxane, tetrahydrofuran, mixtures thereof and the like.

The proportion of solvent is not critical and any proportion may beemployed which will not require excessively large equipment to contain.Generally the weight percent of aromatic nitro compound in the solventis in the range between about 2.0 and about 75 percent, but greater orlesser proportions may be employed if desired.

The reaction temperature is maintained above about C. and preferablybetween about 100 and about 250 C. Interior and/or exterior heating andcooling means may be employed to maintain the temperature within thereactor within the desired range.

The reaction time is dependent upon the aromatic nitro compound beingreacted, and on the amount of catalyst being charged, as well as thetype of equipment being employed. Usually between one-half hour and 20hours are required to obtain the desired degree of reaction in a batchoperation, but shorter or longer reaction times may be employed. In acontinuous process, the reaction time may be much lower, i.e.,substantially instantaneous and residence time may be substantially lessthan batch reaction time.

The reaction can be carried out batchwise, semicontinuously orcontinuously.

After the reaction is completed, the temperature of the crude reactionmixture may be dropped to ambient temperature, the pressure vessel isvented, and the reaction products are removed from the reaction vessel.Filtration or other suitable solid-liquid separation techniques may beemployed to separate the catalyst from the reaction product, andfractional distillation is preferably employed to isolate the aromaticisothiocyanates from the reaction product. However, other suitableseparation techniques such as extraction, sublimation, etc., may beemployed to separate the aromatic isothiocyanates from the unreactedaromatic nitro compound and any by-products that may be formed.

The following examples are presented to describe the invention morefully without any intention of being limited thereby. All parts andpercentages are by weight unless otherwise specified.

- EXAMPLE 1 A 100 ml. rocking autoclave was charged with 4.10 grams(0.033 mole) of nitrobenzene, 0.173 gram (0.0024 mole) of anhydrouspotassium hydrosulfide, and 10 grams of tetramethyl urea. The autoclavewas closed, and to it was charged 21.9 grams (0.365 mole) of carbonylsulfide. The autoclave was then cooled to room temperature and thecontents discharged. The mixture thus obtained Was filtered, and thesolid portion washed with ether. Evaporation of the ether left 11.07grams of filtrate, which was shown by vapor phase chromatography tocontain 1.28 grams nitrobenzene (69 percent conversion) and 1.18 gramsphenylisothiocyanate (38 percent corrected yield).

EXAMPLE 2 A 100 ml. autoclave was charged, as in Example 1, with 4.10grams of nitrobenzene, 0.16 gram of anhydrous KSH and 29.7 grams of COS,and the mixture was heated at 150 C. for 3 hours. The dischargedmaterials was extracted with pentane and a pentane soluble oil (4.0grams) recovered by evaporation of solvent. Analysis of this oil byvapor phase chromatography showed that it contained 2.9 gramsnitrobenzene (29 percent conversion) and 0.84 gram phenylisothiocyanate(66 percent corrected yield).

EXAMPLE 3 A ml. autoclave was charged, as in Example 1, with 4.10 gramsof nitrobenzene, 0.16 gram of anhydrous KSH, and 13.7 grams of COS. Theautoclave was then pressurized with 2600 p.s.i.g. of carbon monoxide,after which it was heated at C. for 3 hours. The resulting product wasextracted with ether, and the ether soluble oil (3.0 grams) was shown byanalysis of vapor phase chromatography to contain 0.1 gram nitrobenzene(97 percent conversion) and 2.2 grams phenylisothiocyanate (51 percentcorrected yield).

EXAMPLE 4 A 100 ml. rocking autoclave, charged with 4.10 grams ofnitrobenzene, 0.48 gram of KSH, and 20 grams of carbon disulfide, washeated at 150 C. for 3 hours. The resulting product was filtered, andthe excess CS evaporated from the filtrate. Analysis of the residual oilby vapor phase chromatography showed it to contain 3.14 grams ofnitrobenzene (23 percent conversion) and 0.57 gram ofphenylisothiocyanate (68 percent corrected yield).

Various modifications of the invention some of which have been referredto above may be employed without departing from the spirit of theinvention.

What is desired to be secured by Letters Patent is:

1. The process for preparing aromatic isothiocyanates which comprisesreacting:

(a) a sulfur-containing compound of carbon selected from the groupconsisting of:

(1) carbon disulfide, (2) carbonyl sulfide, and (3) mixtures of carbondisulfide and carbonyl sulfide, and (b) a carbocyclic aromatic nitrocompound containing between 6 and about 20 carbon atoms in the presenceof (c) anhydrous potassium hydrosulfide as a catalyst at a temperaturebetween about 25 C. and about 250 C. and a pressure between about 30 andabout 2000 p.s.i.g., and recovering the aromatic isothiocyanate producedthereby.

2. The process of claim 1 wherein the molar proportion of potassiumhydrosulfide per mole of nitro groups in said aromatic nitro compound isin the range between about 1:0.3 and about 1:1000.

3. The process of claim 2 wherein the molar proportion ofsulfur-containing compound of carbon per mole of nitro groups in saidaromatic nitro compound is in the range between about 1:1 and about50:1.

4. The process of claim 3 wherein the molar proportion of potassiumhydrosulfide to each nitro group in said aromatic nitro compound is inthe range of between about 1:06 and about 1:100.

5. The process of claim 4 wherein said aromatic nitro compound isselected from the group consisting of nitrobenzene,meta-chloronitrobenzene, nitrotoluene and dinitrotoluene.

6. The process of claim 5 wherein said sulfur compound of carbon iscarbonyl sulfide, and the molar proportion of carbonyl sulfide to saidaromatic nitro compound is in the range of between about 2:1 and about15:1.

7. The process of claim 6 wherein said aromatic nitro compound isnitrobenzene.

8. The process of claim 6 wherein said aromatic nitro compound isdinitrotoluene.

9. The process of claim 6 wherein said aromatic nitro compound isnitrotoluene.

10. The process of claim 5 wherein said sulfur compound of carbon iscarbon disulfide and the molar pro- 8 portion of carbon disulfide tosaid aromatic nitro com- 2,631,167 3/1953 Werner 260689 XR pound is inthe range between about 2:1 and about 15:1. 2,263,386 11/1941 Hester260454 11. The process of claim 10 wherein said aromatic 3,235,5802/1966 Kiihle 260454 nitro compound is nitrobenzene. 3,255,252 6/1966Gold 260689 XR 12. The process of claim 10 wherein said aromatic nitrocompound is nitrotoluene. 5 LEWIS GOTTS, Primary EXamiIler 13. Theprocess of claim 10 wherein said aromatic HOLLRAH Assistant Examinernitro compound is dinitrotoluene.

US. Cl. X.R. References Cited 10 24 302 UNITED STATES PATENTS 1,689,01410/1928 Dieterle 260689 XR

